1. Field of the Invention
The present invention relates to a technique for performing optical frequency rectification so as to more efficiently harvest radiation from the infrared to the visible. More specifically, the present invention relates to a technique of optical frequency rectification based on a geometric asymmetry of the antenna tip, or other shaped edges such as on patch antennas.
2. Description of the Related Art
The uses of rectennas for power transmission and detection, in the microwave region, have a long history. Applications have included: long distance power beaming; signal detection; and, wireless control systems. The first receiving device for efficient reception and rectification of microwave power was developed in the early 1960's.
Point-contact devices (i.e., whisker diodes) have been used in measurements of absolute frequencies up to the green part of the visible spectrum, demonstrating a response time in the order of femto-seconds, which is orders of magnitude faster than with conventional MIM diodes. In addition to the nanowire geometry for the whisker antennas, the use of patch antennas (e.g. microstrip antenna) can have extended solid and open geometries (e.g. squares, rectangles, rings or others) which provide a more robust stability in a practical device.
In addition, the patch antenna can lead to enhanced antenna properties and output (see K. R. Carver and J. W. Mink, IEEE Trans. Antennas and Prop, AP-29, 1, 2 (1981)). Moreover, such antenna arrays of gold have been fabricated on flexible substrates at the Idaho National Laboratory. The planar structure should require simpler fabrication. The technological difficulty of producing arrays of nanometer gap junctions has recently been overcome by Gupta and Willis using atomic layer deposition (ALD). Planar arrays of Cu-vacuum-Cu tunnel junctions were produced on silicon wafers using conventional lithography techniques, followed by ALD to yield tunnel junctions of ˜1 nm.
Recent 3-D quantum mechanical computer simulations of optically irradiated MVM tunnel junctions using Ag, Au and Cu tips predict an enhanced rectification and current output due to the surface plasmonic resonances in these materials at ˜3 eV, corresponding to the energetic green portion of the visible spectrum. Deposition of a thin layer of these metals on an underlying antenna structure such as tungsten, molybdenum or aluminum should yield the same results.
Unlike a conventional planar MIM diode, the rectification process, in the proposed device can be due solely (and/or primarily) to the geometrical asymmetry of the pointed nanowires/mCNTs tip. A razor like edge, produced on a microstrip or other form of patch antenna, can also provide the intrinsic geometric asymmetry necessary for the rectification process.
One of the major challenges in the efficient harvesting of the energy in the optical frequency portion of the spectrum is the development of a broadband device that will rectify from the infrared (IR) through the visible portion of the spectrum ˜1015 Hz, comprising the largest portion of the solar energy spectrum. Currently silicon based energy conversion devices (photovoltaic) are narrow band devices limiting the collection to a lower energy portion of the electromagnetic spectrum.
This technique for optical frequency rectification has applications that includes but are not limited to photovoltaics (the conversion of photon energy to electrical energy), solar cells which convert solar energy to electrical (see U.S. Pat. No. 7,799,988, for an APPARATUS AND SYSTEM FOR A SINGLE ELEMENT SOLAR CELL, issued Sep. 21, 2010, to Paul H. Cutler (hereinafter referred to as Cutler); the entire contents of which is hereby incorporated by reference), thermal or chemical energy, nano-photonics, near field optics, IR sensing and imaging including medical and chemical sensors (see Optical antennas for nano-photonic applications, J. Alda, J. Rico-García, J. López-Alonso, and G. Boreman, Nanotechnology, vol. 16, pp. S230-4, 2005; and, Optical Antennas, Palash Bharadwaj, Brad Deutsch, and Lukas Novotny, Adv. Opt. Photon. 1, 438-483). An additional application is the optical frequency transmission and receiving of information and energy conversion. This is significant since the density of transmitted information is greater at higher frequencies; in fact the density goes as the square of the frequency. For transmission through the atmosphere, losses decrease as the frequency increases
Thus, one of the fundamentally important and critical aspects for harvesting is the ability to achieve optical rectification into the visible portion of the electromagnetic spectrum. With current rectification devices the fastest frequency response is limited to the IR. We have developed a new paradigm for optical rectification and have demonstrated both theoretically and experimentally the feasibility for a long sought practical rectification device in the visible portion of the electromagnetic spectrum.
The prior art has attempted to address a number of the issues raised hereinabove. For instance, U.S. Pat. No. 4,445,050 for a DEVICE FOR CONVERSION OF LIGHT POWER TO ELECTRIC POWER, which issued Apr. 24, 1984 to Alvin M. Marks (hereinafter referred to as Marks-1), proposed a device for the direct conversion of light power to electrical power. The present invention differs from that of Marks-1, in that Marks-1 utilizes a plurality of dipole antennae for absorbing light photons. An alternating electric field of the light photons is employed to cause electrons in the dipole antenna to resonate and absorb electrical power. The DC power is accumulated on conducting busbars from the plurality of antennae and associated rectifying circuits.
Additionally, U.S. Pat. No. 4,720,642 for a FEMTO DIODE AND APPLICATIONS, which issued Jan. 19, 1988 to Marks (hereinafter referred to as Marks-2), discloses a femto-second rectifying device consisting of a sub-micron sized dipole antenna attached to a MIM diode at one end. The Marks-2 device is a traditional planar MIM diode that relies on material choices and not geometry. It is important to note in the MIM rectifying device of Marks-2, the response time of the device will be limited by the choice of materials and does not extend beyond the IR.
Further, U.S. Pat. No. 4,574,161 for an ORDERED DIPOLAR LIGHT-ELECTRIC POWER CONVERTER, which issued Mar. 4, 1986 to Marks (hereinafter referred to as Marks-3), teaches a light to electric power converter comprising a sheet with light-absorbing electrically conducting particles embedded therein. The particles can be metallic, or can be a conductive molecule such as a conjugate carbon chain. The electrodes of Marks-3 are formed in-situ and comprise a salt reduced to a metal, and forming a pre-determined pattern.
What is not appreciated by the prior art is that current silicon based energy conversion devices (photovoltaic) are narrow band devices limiting the collection and conversion to a lower energy portion of the spectrum. In general, conventional rectennas consist of two distinct elements, a dipole antenna plus a separate rectifying device such as an MIM or Schottky diode. Thus, one of the fundamentally important and critical aspects for harvesting is the ability to achieve optical rectification into the visible portion of the solar spectrum. With current rectification devices the fastest frequency response is limited to the IR.
Additionally, another shortcoming of the prior art is the problem of limited frequency response of conventional planar MIM diodes (limited by parasitic capacitance effects).
Cutler was able to address, in great part, the efficiency required for receiving and converting incident radiation into DC current. The present invention expands on the success of Cutler by addressing the need for an improved technique of optical rectification for photovoltaic and other applications based on geometric asymmetry of the antenna tip, or other shaped edges such as on patch antennas. Further, there is a need for the use of point-contact nanowires/mCNTs, and other sharp-edged devices such as patch antennae and their inherent fast response time, to overcome the problem of limited frequency response of conventional planar MIM diodes.